I would definitely recommend, rather than extracting the textures from the map, try instead to contact the map creator/whoever created the textures if you plan to use them in a project you are going to publish publicly. This way you have permission to use them and your not going to get into any trouble for stealing anything.

But if you do want to extract some textures, maybe just for personal use, you can extract them by opening the bsp in GCFscape. Sometimes however you will need to uncompress the bsp first since you can't extract textures from a compressed bsp